What should be paid attention to when cleaning and baking the ultra-high vacuum system?
At present, the main vacuum method to obtain clean ultra-high vacuum is an air extraction system composed of a sputter ion pump and a titanium sublimation pump. The pressure that this vacuum system can obtain is less than 1.3*10⁻³Pa. For small and medium-sized vacuum systems, the main pump is usually a sputtering ion pump; for large vacuum systems, the main pump is usually a titanium sublimation pump. The requirements of the ultra-high vacuum system are:
The cleanliness of the inner wall of the vacuum chamber must be high; the surface of the parts must be smooth; the amount of material outgassed must be small; the parts of the vacuum chamber must be strictly cleaned; the surface of the parts must not be directly touched by hands; strictly control the vacuum chamber Leak rate. In addition, it is necessary to bake a vacuum chamber, the baking temperature range is 200°C~250°C, and the continuous baking time is 10h~12h (or 350°C~400°C baking temperature, continuous baking 4h) .
At present, the domestic magnetron sputtering ion pump has a starting pressure of about 6.7*10⁻¹pa, and it cannot be started at a high pressure. After assembling the sputter ion pump, its first start-up is more troublesome. It is best to remove the magnet and bake the sputter ion pump to achieve the purpose of speeding up the start. The baking temperature of the sputter ion pump is 400°C～500°C, and the continuous baking time is 24h～30h. After thoroughly degassing the pump, it is easier to start. During the baking process of the sputter ion pump, the rough extraction (pre-extraction) pump is used to continuously pump the gas released during the baking.
After the sputtering ion pump has been used for a certain period of time, the ultimate vacuum may drop or it is difficult to start. If after investigation, it is determined that the problem is not caused by air leakage, it may be because the pump is affected by water vapor in the atmosphere or mechanical pump oil vapor. The original performance of the pump can be restored by baking. If it is severely contaminated and the original performance cannot be restored, the pump must be disassembled and the parts inside the pump must be thoroughly cleaned.
In the ultra-high vacuum system, the vacuum chamber generally uses stainless steel as the main material (carbon steel is more susceptible to oxidation, resulting in a higher outgassing rate). Therefore, it is necessary to understand the cleaning work of stainless steel. The purpose of cleaning is to reduce the physical and chemical adsorption of gas on the surface of stainless steel. For the unclean stainless steel surface, it can be observed by the secondary ion spectrometer and Auger spectrometer. It can be found that carbon and its compounds are the main pollutants, and these are all organic matter. Therefore, it is cleaned by various chemical solvents Surface organic matter and carbide are a common and effective cleaning method.
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